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Journal of Nonlinear Optical Physics & MaterialsVol. 12, No. 3 (2003) 335–339c© World Scientific Publishing Company

SALT-INDUCED AGGREGATION AND ENHANCED OPTICAL

LIMITING IN CARBON-BLACK SUSPENSIONS

S. K. TIWARI∗, M. P. JOSHI, S. NATH† and S. C. MEHENDALE

Laser Physics Division, Centre for Advanced Technology,

Indore 452013, India†Radiation Chemistry & Chemical Dynamics Division,

BARC, Mumbai 400085, India∗sanjiv@cat.ernet.in

Received 13 January 2003

Results of a study of optical limiting of 532 nm, 15 nsec laser pulses by carbon-blacksuspensions in pure water and saline water are presented. The optical limiting strengthwas found to increase as the salinity of the host liquid increased. Particle size measure-ments showed that aggregation of carbon particles in the saline suspensions resultedin particles with sizes greater than that for the pure water suspension. As the ther-mophysical properties of the host liquids were nearly the same, the enhanced opticallimiting performance is believed to be a direct consequence of the larger size of thecarbon particles.

Keywords: Optical limiting; carbon-black; aggregation.

Development of optical limiters for protection of the eye and other sensors from

damage by intense laser beams is an area of continuing research interest.1 A large

variety of materials and nonlinear optical processes are being investigated to develop

passive devices for protection from laser pulses with a duration of a few tens of nsec

and shorter. One material which appears particularly promising in this context is

carbon-black suspension (CBS),2 usually prepared by diluting commercial drawing

ink. The attractive features of CBS are low limiting threshold, high low-intensity

transmission and uniform absorption over a wide spectral range which is particu-

larly important for eye protection. It is now established that optical limiting by CBS

is due to nonlinear scattering from micro-bubbles or micro-plasma. For irradiation

by nsec duration laser pulses, scattering from micro-bubbles appears to be the dom-

inant mechanism although the bubble formation itself may be preceded by plasma

formation. As is to be expected, in this case, the thermophysical properties of the

host liquid play an important role in determining the optical limiting strength.3–5

For example, it has been observed that the optical limiting threshold is lower for

∗Corresponding author.

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336 S. K. Tiwari et al.

solvents with lower boiling temperatures because bubble formation can occur at a

smaller value of the incident laser fluence. Another parameter which is expected

to be important in determining the strength of optical limiting is the size of the

suspended carbon particles. There are experimental reports which indicate that

suspensions with larger particles limit better.6–8 However, in these experiments

the change in particle size was accompanied by other changes so that the effect

of size change was not isolated from the influence of other factors. In the experi-

ments reported in Ref. 8, the ink suspensions with particles of different sizes were

prepared using different liquids while the carbon particles with different sizes used

in experiments reported in Ref. 7 were obtained from different sources. The large

carbon particles used in the experiments of Ref. 6 corresponded to an unstable floc

state. In this paper, we present clear evidence of the effect of particle size on optical

limiting by CBS. Stable suspensions of carbon particles were produced by diluting

a commercial black ink in water and saline water with different molarities. Dynamic

light scattering (DLS) measurements showed that the mean size of the aggregates

was larger for saline water suspensions indicating aggregation of the carbon parti-

cles. Optical limiting of 532 nm, 15 nsec duration laser pulses by these suspensions

was studied and the limiting was found to be stronger for the saline suspensions

than for the pure water suspension. As the thermophysical properties of water and

saline water are nearly the same, the observed enhanced optical limiting appears

to be a direct consequence of the size dependence of optical limiting. Our results

thus show that the performance of a CBS-based optical limiter can be optimized

by using particle size as an additional parameter.

Experiments were performed using second harmonic (532 nm, 15 ns, 0.5 Hz) of

the emission from a Q-switched Nd:YAG laser. The laser emission was focused to

a beam waist of ∼ 100 µm at the focal position where the sample cell was placed.

Transmitted pulse energy was measured using a calibrated photo-diode kept after

an aperture (diameter 4 mm, transmission ∼ 0.95 without the sample cell in the

beam path) at a distance of ∼855 mm from the sample cell. Another photo-diode

was used to measure input pulse energy which was varied by using a set of neu-

tral density filters. A drawing ink (M/s Rotring GmbH, Germany) was used as

the source of carbon-black. De-ionized water and aqueous solutions of NaCl were

filtered through a Whatman filter paper of 2.5 µm pore size before making suspen-

sions. Three suspensions of carbon-black were prepared by diluting the ink in pure

water, 0.68 M NaCl solution and 2.04 M NaCl solution. All the suspensions were

sonicated for ∼5 minutes and were found to be stable for several days after ultra-

sonication. For suspensions with molarity greater than ∼ 2.5 M, phase separation

occurred through sedimentation in a short time. The linear transmission spectra

of the samples were recorded using a spectrophotometer (Shimadzu UV-3101 PC)

in the 300–800 nm region. The low intensity transmission of all the samples was

adjusted to be ∼65% at 532 nm. Quantitative estimates of the particle sizes were

obtained using a DLS-based particle size analyzer (Brookhaven Instrument BI-90).

Minimum particle size that can be detected with this instrument is ∼10 nm.

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Salt-Induced Aggregation and Enhanced Optical Limiting 337

0 1 2 30 .0

0 .2

0 .4

0 .6

ou

tpu

t fl

ue

nc

e (

J/c

m2)

i n p u t f lu e n c e ( J / c m2)

Fig. 1. Variation of output fluence with input fluence for CBS in three different host liquids(triangles: pure water; squares: 0.68 M NaCl solution; circles: 2.04 M NaCl solution). The dashedline gives linear transmission.

The linear extinction spectra of CBS in pure water, 0.68 M NaCl solution and

2.04 M NaCl solution were recorded with the spectrophotometer. For the water

and the lower molarity suspensions, the spectra were nearly the same while for

the higher molarity suspension there was a pronounced change in slope indicative

of a change in particle size. For more quantitative information, the particle sizes

were measured using the particle size analyzer. The particle sizes were found to

increase with molarity, with mean diameters of ∼146 nm, 223 nm and 435 nm,

respectively, for the three suspensions. It is well-known9 that addition of a salt to a

suspension results in coagulation of the particles because of a change in the balance

between the attractive van der Waals forces and the repulsive electrostatic forces

due to a decrease in the Debye length. We, therefore, believe that the presence of

NaCl results in aggregation of carbon particles resulting in particles of larger sizes.

Figure 1 shows the measured variation of output fluence with input fluence for

the three suspensions. For the saline suspensions, the departure from low-intensity

transmission occurred at lower fluences and also the output fluence was significantly

lower for high input fluences. The limiting threshold (defined as the input fluence

at which the transmittance falls to 50% of the linear transmittance) for the pure

water, 0.68 M saline and 2.04 M saline suspensions were, respectively, ∼0.9 J/cm2,

0.75 J/cm2 and 0.5 J/cm2.

As mentioned earlier, optical limiting of nsec duration laser pulses by CBS is

believed to be mainly due to nonlinear scattering from micro-bubbles produced

by solvent evaporation following heat transfer from the hot absorbing particles.

The thermo-physical properties of the host liquids are nearly the same for the

suspensions used in our experiments; the values of boiling point (◦C), thermal con-

ductivity (W/m◦K) and specific heat (J/kg◦C) for water and sea water (average

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338 S. K. Tiwari et al.

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 00 . 0

2 . 0

4 . 0

6 . 0

8 . 0

[Eabs/V

]X10

-4 (J/c

m3)

p a r t i c l e r a d i u s ( n m )

- 1 3 . 5

- 1 2 . 0

- 1 0 . 5

- 9 . 0

- 7 . 5

log

10(E

abs)

Fig. 2. Calculated variation of the energy absorbed per unit volume in J/cm3 (continuous curve)and the total absorbed energy in J (dashed curve) for a spherical carbon particle with the particleradius. The laser fluence is 0.5 J/cm2.

salinity ∼ 0.6 M) are 100, 0.591, 4181.6 and 100.58, 0.596, 3993, respectively.10

Thus the observed large differences in the strengths of optical limiting appear to

be a direct consequence of the change in particle size. We calculated the energy

absorbed per unit volume Eabs/V — which is proportional to the temperature rise

on heating — and the total energy absorbed by a carbon particle Eabs on irra-

diation by a laser pulse using the program BHMIE from Bohren and Huffman.11

This calculation assumes a spherical particle and heat transfer to the surrounding

medium is neglected. The complex refractive index of carbon at 532 nm was taken

as 1.95+i 0.66 (see Ref. 12) and that of water as 1.33. Figure 2 shows the calculated

variation of the energy absorbed per unit volume and the total absorbed energy

with particle radius for an input fluence of 0.5 J/cm2. For the particle sizes under

consideration, it is noteworthy that the temperature rise, in fact, is expected to

decrease with increasing size while the total absorbed energy increases. The reason

for this behavior is that for particle sizes larger than the absorption length

(∼130 nm for carbon at 532 nm), the absorbed energy increases only as the square of

radius. Thus the observed stronger optical limiting for suspensions with larger par-

ticles is not because these particles get heated to a higher temperature but appears

to be due, as suggested in Ref. 7, to a stronger local heating of the surrounding

fluid because the larger particles contain more energy individually. The heat trans-

fer to the surrounding fluid is also expected to be more efficient for larger particles

because of larger surface area. Additionally, the scattering may also be enhanced

because larger micro-bubbles can form rapidly around the hot particle by spinodal

decomposition.13 It would be worthwhile to investigate how far this trend of bet-

ter limiting with larger particles continues and whether there exists an optimum

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October 15, 2003 17:16 WSPC/145-JNOPM 00147

Salt-Induced Aggregation and Enhanced Optical Limiting 339

size. Investigating a larger range of particle sizes may require a different strategy,

possibly forming suspensions starting with carbon particles of different sizes.

To conclude, results of a study of optical limiting of 532 nm, 15 nsec duration

laser pulses in carbon-black suspensions in pure and saline water were presented.

Dynamic light scattering studies showed that the particles were larger in saline sus-

pensions indicating aggregation of carbon particles. Optical limiting was observed

to be stronger for the saline suspensions than for the water suspension. As the ther-

mophysical properties of the suspensions were nearly the same, our results provide

a clear demonstration of the particle size dependence of optical limiting by CBS.

Acknowledgment

We wish to thank K. C. Rustagi for helpful discussions and a critical reading of the

manuscript.

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